Recent Advances in the Use of Vibrational Chiroptical Spectroscopic Methods for Stereochemical Characterization of Natural Products
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Natural Product Reports Recent advances in the use of vibrational chiroptical spectroscopic methods for stereochemical characterization of natural products Journal: Natural Product Reports Manuscript ID: NP-REV-02-2015-000027.R1 Article Type: Review Article Date Submitted by the Author: 22-May-2015 Complete List of Authors: Bolzani, Vanderlan; São Paulo State University (UNESP), Institute of Chemistry Batista Junior, Joao Marcos; Federal University of Sao Carlos - UFSCar, Chemistry Blanch, Ewan; RMIT University, School of Applied Sciences Page 1 of 20Journal Name Natural Product Reports Dynamic Article Links ► Cite this: DOI: 10.1039/c0xx00000x www.rsc.org/xxxxxx ARTICLE TYPE Recent advances in the use of vibrational chiroptical spectroscopic methods for stereochemical characterization of natural products João Marcos Batista Jr.,* a Ewan W. Blanch b and Vanderlan da Silva Bolzani*c Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX 5 DOI: 10.1039/b000000x This review covers conformational and configurational assignments in natural product molecules using chiroptical spectroscopy reported over the last 15 years. Special attention is given to vibrational optical activity methods associated with quantum mechanical calculations. Covering: 2000 to 2014 ROA, on the other hand, results from a small difference in the Introduction intensity of vibrational Raman scattering from chiral molecules when using right- and left-circularly polarized incident light 10 Chirality is omnipresent in nature manifesting itself in all 1 50 (ICP-ROA). It can be equivalently measured as the difference in material objects ranging from fundamental particles to galaxies. intensity of a small circularly polarized component in the When it comes to natural product chemistry, molecular chirality, scattered light by using incident light of fixed nonelliptical that is, the chiral three-dimensional arrangement of atoms in a polarization (SCP-ROA). given molecule, is of paramount importance to the understanding 2 OR, ORD, ECD, VCD, and ROA, which are all inherently 15 of biosynthesis, functions and biological activities. However, 55 sensitive to chirality, are nondestructive methods that can be characterizing such arrangements in the absolute sense is not a measured directly in solution, without the need of crystallization trivial task. Enantiomers share, in an achiral environment, or the use of chiral auxiliaries. Additionally, due to their basically the same physical and chemical properties that make sensitivity to molecular conformations, not only is absolute them indistinguishable in many aspects. One way of getting stereochemistry determination feasible but also conformational 20 around this problem is to take advantage of the diastereomeric 60 analysis in solution. Nevertheless, the interpretation of chiroptical discrimination provided by chiroptical methods. 3 data in order to obtain molecular structure information is not Although the term “chiroptical” (Chiral + Optical) may not be straightforward. Although the so-called ECD “exciton chirality” readily recognizable to all natural product chemists, this concept method (ECM) 6 has been successfully used for many years to is commonly present in their everyday lives. Optical rotation determine the absolute configuration of suitable molecules, only 25 (OR, polarimetry), one of the oldest chiroptical methods 65 recently, due to the development of reliable ab initio calculations available, is widely used in teaching, analytic, synthetic and for predicting theoretical spectra, the use of chiroptical methods natural product laboratories, mainly to determine enantiomeric 4 for configurational analysis of chiral molecules faced a purity. Other examples of chiroptical methods include optical 7 renaissance. rotatory dispersion (ORD), electronic circular dichroism (ECD), This renewed interest in chiroptical spectroscopy methods 30 and vibrational optical activity (VOA) methods, represented by 70 involved not only VOA techniques. The development of the vibrational circular dichroism (VCD) and Raman optical activity accurate theoretical calculations mentioned above also greatly (ROA). The latter two will be the focus of the present review expanded the applicability of OR/ORD and ECD for structural article. characterization of natural products, with a great number of All chiroptical methods arise from the differential interaction reports published every year. Interestingly, these calculations 35 of a chiral non-racemic molecule with left- and right-circularly 5 75 have recently revealed the first apparent exception of ECM for a polarized light beams, which are chiral entities with one being simple biaryl system. 8 Although not the focus of this review the mirror image of the other. OR and ORD (variation of OR with article, excellent references on the use of OR/ORD and ECD wavelength) are based on different indices of refraction for left- techniques for synthetic and natural organic molecules can be and right-circularly polarized light in a chiral (optically active) 5,9-27 found here. 40 medium, known as circular birefringence. As light traverses a 80 Compared to the other chiroptical methods, VCD and ROA chiral medium, the circularly polarized electromagnetic field present some advantages, since they can be measured for virtually components slow to a different extent and the plane of any molecule without the requirement of either UV-Vis polarization is rotated with respect to the original case. Circular chromophores for ECD, derivatizations, or simulations of dichroism methods originate from the differential absorption by a excited-state wavefunctions. In addition, as these two 45 chiral molecule of left- versus right-circularly polarized radiation 85 spectroscopic methods represent global molecular properties, all during an electronic (ECD) or vibrational transition (VCD). degrees of freedom (3N-6, where N is the number of atoms) of a This journal is © The Royal Society of Chemistry [year] [journal] , [year], [vol] , 00–00 | 1 Natural Product Reports Page 2 of 20 molecule can be investigated, at least in theory. Nevertheless, at consists of extracting dipole and rotational strengths from the same time, it becomes more difficult to extract stereochemical experimental data and plotting it against the calculated values. information from VOA spectra, which increases the dependence 60 This method offers the possibility of calculating statistical on the accuracy of the calculations. VCD and ROA also require measures, such as the correlation coefficient R 2.30 A second 44 5 larger amounts of sample (~ 0.01-0.1M) and longer acquisition method called SimIR/VCD uses computationally optimized times (~ 1-12 h) than do OR/ORD and ECD. frequency scaling and shifting to match calculated and observed Despite the advantages described above, an early review article spectra. A third method is the confidence level algorithm 45 that 28 on chiroptical methods published by Allenmark in 2000, 65 provides a direct quantitative comparison of the experimental presented only a single example on the use of VCD for natural spectrum with the calculated spectra for both enantiomers as a 10 products (the Scolytidae insect pheromone frontalin). Fortunately, measure of the degree of agreement and hence level of this scenario has been changing rapidly and a number of review confidence. The most recent method is based on the similarity of articles and book chapters have already been published dealing dissymmetry factor spectra placing emphasis on robust regions 46 with the use of VOA to natural product chemistry, although 70 both in the experimental and calculated spectra. focused mainly on VCD. 5,25,29-36 Recently, there have also been some attempts to interpret VCD 15 The present review will report approximately 190 molecules data without the aid of theoretical calculations, which resulted in whose stereochemical properties have been studied using VOA the development of the exciton chirality method for VCD. 47 methods, including VCD and ROA, over the last fifteen years. Briefly, this methodology is based on the coupling of two We hope this comprehensive review will demonstrate the 75 infrared chromophores (e.g. carbonyl groups) that yields a strong evolution of the applications of VOA to natural product VCD couplet whose sign reflects the absolute configuration of 20 chemistry, and help spread the word about the potential of these the molecule. methods to solve stereochemical problems of chiral natural However, it is important to emphasize that, even when DFT molecules. calculations are employed to generate theoretical spectra, the 80 assignment of absolute configurations may be inaccurate. This VOA techniques may be caused by errors in the equilibrium geometries and conformational populations as mentioned before, by density VCD and ROA were first measured in the early-to-mid-1970s, 37- 39 functional and/or basis set errors, and finally by solvent effects. 25 and, ever since then, VOA has undergone tremendous Density functional and basis set errors are expected to develop in evolution that culminated in the availability of user-friendly 85 any quantum mechanical calculation because it is based on commercial instrumentation 40 and software packages 41 for approximations of atomic wave functions, the accuracy of such reliable spectral measurement and quantum mechanical predictions being sensitive to the choice of the functional (either calculations of theoretical spectra. The determination of the pure or hybrid), and directly proportional